The production of sintered parts from "green bodies" is well known in the prior art. Generally, the green body is formed by filling a die with a powder/binder mixture and compacting the mixture under pressure. The green body, which is a self-supporting structure, is then removed from the die and sintered. During the sintering process, the binder is volatilized and burned out. The sintered part is typically not fully dense (approximately 85-90% of theoretical density) and has substantially reduced mechanical properties compared to wrought material.
Injection molding is a preferred process for manufacturing complex, near-net shape parts from metal and ceramic powders. The tools for producing injection molded parts are generally made from metals, such as aluminum and tool steels in particular, due to the high pressures and temperatures normally required for state-of-the-art metal and ceramic feedstocks. Construction of hard tooling frequently involves long manufacturing turnaround times and is very costly. Furthermore, determination of accurate shrinkage factors for the tool material typically requires multiple iterations of successive, expensive machining.
Most state-of-the-art molding formulations use organic waxes and/or polymers as the molding, fluidizing medium. Polymer-based molding formulations require high molding pressures up to about 15,000 psi and molding temperatures above 212.degree. F. (R. M. German and A. Bose, "Injection Molding of Metals and Ceramics," Metal Powder Industries Federation, p53, p144, Princeton, N.J. 1997). The high pressures and temperatures employed by these systems necessitate the use of metal tooling, such as aluminum and steel, and preclude the use of "soft" tooling, such as organic polymer-based materials. The use of such soft tooling, utilizing plastic resins which are usually fortified with a particulate filler, obviates much of the high cost of manufacture and long turnaround times associated with hard tools. Furthermore, soft tooling can be more easily modified than can hard tooling, thus facilitating more accurate determination of shrinkage factors for net shape fabrication of metal and ceramic parts.
The advantages of soft tools for net shape fabrication can be realized using injection molding feedstock materials which permit the use of low molding pressures below about 1500 psi and low temperatures below about 212.degree. F.
Typical examples of these feedstock materials are aqueous, polysaccharide-based injection molding compounds.